In the screening of fluidizable suspensions, such as medium consistency paper pulp, it is desirable to utilize a simple yet effective apparatus, and one which will allow the production of a number of different accept streams. Medium consistency pulp typically is pulp having a consistency of about 6-15 percent, and most typically within the range of about 8-12 percent solids. In order to effectively screen such suspensions, it is necessary to fluidize them by producing pulsations and shear forces in the suspension. When the suspension is fluidized the fibers or other solid materials therein move relative to each other, and can be separated according to different sizes, shapes, or weights utilizing an appropriate screening apparatus.
In co-pending application Ser. No. 836,123 filed Mar. 4, 1986 (the disclosure of which is hereby incorporated by reference herein) a simple yet effective screening device which is particularly adapted for screening medium consistency pulp (although it may be utilized in the screening of other fluidizable suspensions) is illustrated. That screening device is typically utilized with chemical pulps, or mechanical pulps, such as TMP, CTMP, or the like. While the screening device illustrated therein is eminently practical, it has been found according to the present invention that such a device may be made even more useful by effecting reduction of shives, and other components of the rejects, within the screening device housing itself.
According to the present invention, for a screening device such as illustrated in said co-pending application Ser. No. 836,123, or a like screening device having a rotor which is rotatable with respect to a screen, rejects refining is provided directly in the screen housing itself by providing first and second grinding elements downstream of the screening means, that is between the screens and the rejects outlet from the housing. The grinding elements comprise a first element operatively mounted to the housing, and a second element operatively mounted to, and rotatable with, the rotor. The grinding elements preferably are in the form of rings which are removably attached to the housing and the rotor, respectively, and define a grinding slot between them. The dimensions of the grinding slot may be adjusted by replacing one or both of the annular grinding elements, or by forming the grinding elements so that they have a conical shape and adjusting the axial position of the rotor within the housing.
It has also been found, according to the present invention, that further shive reduction can be effected by utilizing vanes attached to the rotor, downstream of the grinding elements, which vanes simultaneously effect pumping of the rejects out of the main outlet. The vanes are as described in said co-pending application Ser. No. 836,123.
According to the present invention a method of treating a suspension of finely comminuted cellulosic fibrous material also is provided. The method comprises the following steps: (a) Introducing suspension having a consistency between about 6-15 percent into the housing inlet. (b) Effecting fluidization of the suspension within the housing to effect screening thereof, at least one accept stream being discharged from the housing through said at least one accepts outlet. (c) During screening of the suspension, effecting thickening of the suspension so that the suspension is thickened to a consistency of between about 15-20 percent. And (d), effecting reduction of the reject material in the thickened suspension prior to discharging the reject material out of the main outlet. Step (d) is practiced by passing the rejects through a grinding slot defined by the relatively rotatable grinding elements. Further shive reduction, and powered discharge of the rejects from the housing, is practiced by supplying dilution liquid to the suspension after grinding so that it is diluted to a consistency of between about 6-15 percent, and then acting on the suspension with rotating vanes (which effect the further shive reduction and pump the rejects out of the housing). Further reject reduction can be provided exterior of the housing, too, with a significant reduction in the energy expended in practicing the subsequent refining to get the desired results.
It is the primary object of the present invention to provide a device and procedure which are eminently suited for effecting screening of paper pulp (preferably medium consistency), or the like, while additionally effecting reduction of shives and other rejects material. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.